1. Field of the Invention
The present invention relates to a method of making an integrated circuit inductor.
2. Description of the Related Technology
An integrated circuit inductor is an inductive component built up on a substrate in a similar manner to other components, and generally includes a spiral conductive trace. This arrangement has the disadvantage of high parasitic loss, in the form of resistive and capacitive loss to the semiconductor substrate on which the integrated circuit is built.
A solution to this problem is to selectively etch the silicon substrate beneath the conductive trace, as described in U.S. Pat. No. 5,539,241. This solution is shown in the accompanying drawings.
FIG. 1 is a plan view of an integrated circuit inductor 1 according to the prior art. FIG. 2 is a cross-sectional view of the inductor 1 on the line II—II in FIG. 1.
Referring to the figures, the inductor 1 comprises a spiral aluminum track 2 is laid down on an insulating oxide layer 3, which itself covers a semiconductor substrate 4. An underpass track 6 at a lower level in deposited layers making up the inductor 1 stretches from an inner end 5 to a connection 7. An outer end 8 is directly connected to another connection 9 at the same level. The spiral track 2 has a substantial extent. If the spiral track 2 is spaced from the semiconductor substrate 4 by a thickness of the insulating layer 3 alone, which has a comparatively high dielectric constant, the inductive track 2 has an appreciable distributed capacitance to the substrate 4. Hence, at high frequencies, the track 2 is liable to lose its inductive effect and acts as a capacitor.
In accordance with the improvement of U.S. Pat. No. 5,539,241, the oxide layer 3 has apertures 10 formed in it, with a track supporting portion 11 connected to the surrounding layer by bridges 12. Via the apertures 10, part of the substrate 4 is etched away, leaving an air gap 14 beneath the oxide portion 11. Thus, the inductive track 2 is widely spaced from the substrate 4, reducing the distributed capacitance thereto since air has a low dielectric constant.
Whilst this is a distinct improvement, the formation of the air gap 14 requires the use of aggressive chemicals and is made by a slow process. Further, the air gap 14 can be formed only in a post-fabrication step, because a liquid etchant required is not used in a CMOS fabrication. Accordingly, there is a need for an improved integrated circuit inductor and a method of producing thereof.